In present-day data processing systems, it is the usual practice to employ magnetic memory disks for storing binary bits representing digital data. The memory disks usually comprise a magnetic disc which is scanned by a magnetic transducer head. The magnetic head is capable of inducing flux reversals in the magnetic domains of the disk and, in turn, of reading a pattern of magnetic orientations on the disk, and translating changes in the magnetic orientation into a series of digitally encoded binary bits.
Several types of magnetic head/magnetic memory disk interfaces are used in present-day data processing systems. For example, magnetic tape memories and floppy magnetic disk memories include magnetic heads which are in intimate contact with the magnetic memory. Another type of magnetic memory is known as the Winchester type which uses rigid magnetic disks. The Winchester magnetic disk memory provides maximum reliability and minimum error generation by eliminating physical contact between the magnetic head and the magnetic disk. This is achieved by means of a flying magnetic head which does not actually contact the surface of the magnetic disk.
It is evident that for maximum efficiency it is essential that the actual displacement of the head from the surface of the magnetic disk be kept at a minimum. Present-day systems are available in which the displacement is of the order of 10-14 microns. Accordingly, for satisfactory operation of the Winchester system it is essential that the surface of the magnetic disk be extremely flat and uniform.
The magnetic disk for the Winchester system is currently prepared from a slurry of gamma ferric oxide mixed in a matrix of an organic material capable of forming a thin uniform magnetic film. A rigid disk was used and the magnetic film deposited on the disk was burnished to provide the uniform surface characteristics required in that type of drive.
U.S. Pat. No. 3,634,047 discloses a method and apparatus for electroplating the magnetic film on the disk substrate so as to provide a magnetic memory disc suitable for use in the Winchester system. However, prior to the electroplating of the main magnetic film, the practice is to provide a fine grain paramagnetic film. This is usually achieved by electroless deposition techniques of a film of paramagnetic nickel/phosphorous material. However, prior to the electroplating of the main film, it is necessary for the paramagnetic nickel/phosphorous film in accordance with the prior art techniques to be burnished and polished so as to remove some of the nodules that result from the electroless deposition process.
U.S. Pat. No. 3,634,209 describes a process for producing magnetic memory devices in which the nickel/phosphorous fine grain paramagnetic film is deposited on the substrate by electroplating means, and in which the main magnetic film is then electroplated over the paramagnetic film. However, again, in order to achieve the uniform density required for the Winchester-type of system, the paramagnetic nickel/phosphorous film must be polished and burnished prior to electroplating the main magnetic film.
The requirement for burnishing and polishing in the prior art methods is primarily due to the difficulty of maintaining constant current densities over the entire plating surfaces of the disk during electroplating. In particular, since the thickness of an electrodeposit at any point on a plateable surface is proportional to the time integral of the current at that point, and since the magnetic properties of a deposit varies somewhat with the current density developed during electroplating, the lack of close control over current density in conventional electroplating apparatus has made it very difficult to plate magnetic surfaces capable of high density recording.
Accordingly, the prior art approach to provide a magnetic memory disk capable of high density recording and suitable for use in a Winchester system usually involves the following steps:
(1) An aluminum substrate is prepared by stamping a plate into the proper pre-defined dimensions. Standards have been defined by the American Society for testing materials for disks of fourteen inch, eight inch, five and one-quarter inch and 3.1 inch outer diameters.
(2) The substrates are then machined and stress relieved to obtain the finest tolerances possible.
(3) The substrates are then diamond turned and/or polished to an extremely fine finish.
(4) The polished substrates are then subjected to a series of plating operations to place a thin film of fine grain paramagnetic nickel/phosphorous material over the surface of the substrate. This film may be of the order of 0.0002 inches thick. The film may be deposited on the polished substrates either by electroless deposition techniques, or by electroplating as described in the Wolf patent.
(5) The coated substrates are then polished again in an effort to remove some of the nodules that result from the deposition process.
(6) After the polishing operation, the disks are reracked and subjected to an electroplating operation, for example, such as described in the Faulkner patent, so that the main magnetic film may be deposited over the paramagnetic film with the required overall degree of uniformity.
(7) A protective barrier coating may then be formed over the surfaces of the plated disk.
The prior art methods, as described above, are relatively expensive, especially in the requirements of the polishing and burnishing operations. These operations are usually performed manually, and are the leading causes for product failure.
Additional problems occur when the paramagnetic nickel/phosphorous film is deposited by electroless methods due to slight variations in the characteristics of the film over the surface of the disk. These variations result in major changes in the signal response during read/write operations.
An important objective of the present invention is to provide a method and process by which the paramagnetic nickel/phosphorous film may be deposited on the substrate by electroplating techniques, so as to obviate the problems encountered when electroless deposition is used, and by which the paramagnetic film is provided with a high degree of uniformity so as to eliminate any need for the time consuming and expensive manual polishing and burnishing operations.
A detailed description may be found in the prior art with regard to the inducement of the growth of grains in an electroplated metal film and the nucleation on the film surface. Once a stable nucleus is established, the surrounding atomic steps (actually lattice grains) are energetically favored sites of high binding energy for further atoms to deposit. Growth can thus occur at levels well below the absolute barrier energy of the surface until the layer is complete. Since an abundance of surface flaws exists, providing the necessary critical energies for lattice formation, the initiation of growth on the surface by electrochemical reaction can proceed without undue duress. This activity is demonstrated in any electroplating operation.
The growth of the metallic electroplated film is fractal with random curdling in a plane grid. Each cascade stage replaces a curd with a certain number of subcurds. Not only are the positions of the subcurds random, but their numbers and their resulting distributions involve classic birth and death processes. At each stage, each curd edge can be viewed as having acquired a random offspring made of the subcurd edges. The classical results on birth and death processes show that the number N(m) of the offspring subcurds present after the m'th generation are determined by the following alternative:
When &lt;N.sub.1 &gt;.ltoreq.1, that is D.ltoreq.2, it is almost certain that the offspring will eventually die out, meaning that the edge will eventually become empty, and hence of zero dimension. On the other hand when &lt;N.sub.1 &gt;&gt;1, that is, D&gt;2, then the offspring subcurd will have a less than one probability of dying off and a non-zero probability of expanding in numbers forever. D represents a similarity dimension in describing factal interactions.
Accordingly, the following asymptotic relationship holds true: ##EQU1##
The foregoing relationship suggests a generalized similarity dimension D-2.
The two-dimensional eddy traces obey an obvious modification of the same analysis, after replacing N.sub.1 by a random N.sub.2 such that &lt;N.sub.2 &gt;=Nr. When (N.sub.2).ltoreq.1, that is, D.ltoreq.1, each eddy face will eventually become empty. When (N.sub.2)&gt;1, that is D&gt;1, it can be demonstrated that: ##EQU2##
Hence the actual action of lattice growth in the electrodeposited paramagnetic nickel/phosphorous film is a competitive rate between a non-zero random curd fractal and the lattice deposition equilibrium of the structure. By observation, the competitive reaction results in an irregular film. Therefore, the biasing of the curdling in favor of the lattice energy results in a truly uniform paramagnetic film.
The paramagnetic film is composed of fine grain magnetic material and it consists of 92-88% nickel and 8-12% phosphorous. To achieve a truly uniform paramagnetic nickel/phosphorous film by electroplating, the electroplating process must be controlled so as to provide nucleation energy for electro-deposition on the flawed lattice areas, and by then providing for a non-zero curdling growth for a predetermined period, followed by a forced curdling death, leaving enough randomly oriented nucleation sites for the next growth cycle. In the practice of the present invention, the foregoing is achieved by providing an apertured rotating mask in the electroplating apparatus between the anode and the target electrode so that the periodicity of the uniform current flow in the electrolyte is maintained. The rate of deposition is a function of the cycling period of the mask, and the average grain size and distribution of the grain size can be readily controlled.
The use of the apertured mask obviates any need for burnishing or polishing operations and reduces the entire manufacturing process from several hours to about 20 minutes. The aperture in the rotating mask insofar as the paramagnetic layer is concerned is oriented to follow the radius line of the target electrode, and it provides a uniform current density across the surface of the target electrode as it rotates because each increment of the surface area sees the same current. A second apertured mask may be used for electroplating the magnetic layer over the paramagnetic layer in which the aperture has parallel sides so as to cause the magnetic layer to be slightly thicker towards the center of the disk for uniform magnetic response over the entire disk surface. Also, as will be described, two masks may be used and multiple apertures for improved results.